Method for producing semiconductor device

ABSTRACT

In irradiating and scanning an amorphous silicon film formed on the glass substrate with a linear laser beam, the glass substrate is placed so as to assume a convex surface. In a heated state, the amorphous silicon film is irradiated and scanned with the linear laser beam having an inverted-U-shaped focus line that approximately coincides with the convex surface. Slow cooling is thereafter performed. A silicon film having uniform crystallinity is formed on a glass substrate having flat surface. Also, A thin film transistor (TFT) having a uniform threshold voltage is produced by using the crystalline silicon film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor producing method offorming a highly uniform crystalline silicon film in processes relatingto manufacture of insulated-gate semiconductor devices such as thin-filmtransistors (TFTs) which are formed by using a non-single-crystal,crystalline silicon film provided on a glass substrate, and othersemiconductor devices. In particular, the invention is effective informing a semiconductor device on a glass substrate.

2. Description of the Related Art

In recent years, insulated-gate field-effect transistors having athin-film active layer (or active region) on an insulative substrate,i.e., thin-film transistors (TFTs), have been studied eagerly.

The TFTs are classified into an amorphous silicon TFT, a crystallinesilicon TFT, etc. by the semiconductor material used and its crystalstate. The "crystalline silicon" does not always mean single crystalsilicon but may mean non-single crystal silicon in some cases. TFTsusing the latter are generally called non-single-crystal silicon TFTs.

In general, amorphous semiconductors have a small electric fieldmobility, which therefore cannot be used for TFTs that are required tooperate at high speed. Further, amorphous silicon can provide only avery small P-type electric field mobility, it does not allow formationof a P-channel TFT (i.e., PMOS TFT), so that a complementary MOS (CMOS)circuit cannot be formed by combining P-channel TFTs and N-channel TFTs(NMOS TFTs).

In contrast, since crystalline semiconductors have larger electric fieldmobilities than amorphous semiconductors, they allow high-speedoperation of TFTs. Further, allowing formation of not only an NMOS TFTbut also a PMOS TFT, crystalline silicon enables formation of a CMOScircuit.

A non-single-crystal silicon film is obtained by forming an amorphoussilicon film by vapor-phase growth and then thermally annealing it for along time at a proper temperature (usually higher than 600° C.) orirradiating it with strong light such as laser light (opticalannealing).

However, where a glass substrate, which is inexpensive and highlyworkable, is used as an insulative substrate, it is very difficult toform, only by thermal annealing, a crystalline silicon film having asufficiently large electric field mobility (to allow formation of a CMOScircuit). This is because a glass substrate generally has a low straintemperature (about 600° C.) and therefore it is distorted when itstemperature is increased to a value that is necessary to form acrystalline silicon film having a sufficiently high mobility.

On the other hand, where optical annealing is used to crystalline asilicon film formed on a glass substrate, high energy can be applied toonly the silicon film without much increasing the temperature of thesubstrate. The optical annealing is thus very effective forcrystallization of a silicon film formed on a glass substrate.

At present, large output pulsed lasers such as excimer lasers areconsidered most suitable for a light source for optical annealing. Sincethose lasers have much larger maximum energies than CW lasers such as anargon ion laser, they allow use of a beam spot as large as severalsquare centimeters, thus contributing to increase of the productivity.

However, to process a large-area substrate with an ordinary square orrectangular beam, it is necessary to move the beam in the two orthogonaldirections. This is an item to be improved to increase the productivity.

This item can be greatly improved by deforming a beam into a linearshape that is longer than the width of a substrate to be processed andscanning the substrate with the beam relatively. (The scanning isperformed by moving a linear laser beam in small steps with overlaps.)Details are described in Japanese Unexamined Patent Publication No.5-112355.

A crystalline silicon film having a higher degree of crystallinity canbe obtained by performing thermal annealing before the opticalannealing. As for the method of thermal annealing, as disclosed inJapanese Unexamined Patent Publication No. 6-244104, a crystallinesilicon film can be obtained at a lower temperature and in a shortertime than the case of using ordinary thermal annealing by utilizing thefact that such elements as nickel, iron, cobalt, platinum, palladium(hereinafter referred to as "crystallization catalyst elements" orsimply "catalyst elements") have an effect of acceleratingcrystallization of amorphous silicon.

TFTs were formed in matrix form by using a crystalline silicon filmformed by a conventional method in which an amorphous silicon film wasformed on a glass substrate, annealed, and then subjected to laserannealing with a linear laser beam, and a distribution of theirthreshold voltages in the substrate surface was examined, which is shownin FIG. 2. It is seen from FIG. 2 that the distribution assumes aU-shape.

FIG. 4 shows an arrangement of TFTs on a glass substrate. In FIG. 4,TFTs are arranged in a matrix of 400×300 in an area of 40 mm×50 mm of a100 mm×100 mm Corning 7059 substrate. In the data of FIG. 2, thehorizontal axis shows positions of 400 TFTs on a horizontal full row(enclosed by a broken line in FIG. 4) of the substrate at the center inthe vertical direction.

If TFTs of a pixel matrix that constitutes a pixel area of a liquidcrystal display has the distribution of threshold voltages as shown inFIG. 2, there may occur display unevenness or an image defect.

An investigation into the cause of the above U-shaped distribution ofthreshold voltages in the substrate surface has revealed that it is verysimilar to a warp in a substrate immediately before application of laserlight.

It has also been found that a glass substrate does not have a warpimmediately after formation of an amorphous silicon film thereon and awarp occurs due to the fact that the silicon film contracts more thanthe glass substrate when the substrate is cooled after a heat treatmentfor crystallizing the amorphous silicon film by solid-phase growth. Thewarp occurs so as to be concave toward the film forming surface of asubstrate.

FIG. 3 shows how laser annealing is performed with linear laser light 2on a silicon substrate formed on a warped glass substrate 1. In FIG. 3,if the warped substrate 1 is subjected to laser annealing, the substratesurface deviates from focuses 3 of laser light differently at respectivepositions. It is considered that these deviations cause the silicon filmto have different degrees of crystallinity in the substrate surface, sothat threshold voltages exhibit a particular distribution in thesubstrate surface.

In the substrate with which the data of FIG. 2 was obtained, the overalldepth of the U shape of the warped substrate was about 50 μm immediatelybefore the application of laser light.

The degree of warp depends on the temperature and time of the heattreatment, the substrate material, and other factors. In the case of a100 mm×100 mm substrate, the depth of the U shape generally fell withinthe range of 20 to 200 μm.

A concave warp occurred not only after the thermal crystallization of anamorphous silicon film formed on a glass substrate but also after slowcooling that was performed after an amorphous silicon film was subjectedto laser annealing while being heated.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for forming acrystalline silicon film on a glass substrate which film has a uniformdistribution of crystallinity in the substrate surface.

Another object of the invention is to provide a method for formingcrystalline silicon thin film transistors (TFTs) on a glass substratewhich TFTs have a uniform distribution of threshold voltages in thesubstrate surface.

Another object of the invention is to provide a producing method whichprovides a uniform distribution of crystallinity in the substratesurface in a process of forming a crystalline silicon film on a glasssubstrate which process includes a thermal annealing step and asubsequent laser annealing step, and which method forms crystallinesilicon TFTs having a uniform distribution of threshold voltages in thesubstrate surface by using the silicon film thus obtained.

Another object of the invention is to obtain a flat substrate after slowcooling that is performed after an amorphous or crystalline silicon filmis subjected to laser annealing while being heated.

To attain the above objects, according to the invention, there isprovided a producing method of a semiconductor device, wherein a linearlaser beam that is applied to an uneven irradiation surface on which asemiconductor film is formed has a focus line that extends in alongitudinal direction of the linear laser beam and approximatelycoincides with a sectional shape of the irradiation surface.

According to the invention, there is provided a producing method of asemiconductor device, wherein in irradiating and scanning asemiconductor film formed on an uneven surface with a linear laser beam,the linear laser beam has a focus line that extends in a longitudinaldirection thereof and approximately coincides with a sectional shape ofthe irradiation surface.

According to the invention, there is provided a producing method of asemiconductor device, comprising, in irradiating and scanning anamorphous silicon film formed on a glass substrate with a linear laserbeam, the steps of: placing the glass substrate so as to have a convexsurface; irradiating and scanning, in a heated state, the amorphoussilicon film with the linear laser beam having an inverted-U-shapedfocus line that approximately coincides with the convex surface; andperforming cooling.

According to the invention, there is provided a producing method of asemiconductor device, comprising, in converting an amorphous siliconfilm formed on a glass substrate into a crystalline silicon film byheating and irradiating and scanning the crystalline silicon film with alinear laser beam, the steps of: placing the glass substrate so as tohave a convex surface; irradiating and scanning, in a heated state, thecrystalline silicon film with the linear laser beam having aninverted-U-shaped focus line that approximately coincides with theconvex surface; and performing cooling.

In the above producing methods, as the amorphous or crystalline siliconfilm is irradiated and scanned with the linear laser beam having theinverted-U-shaped focus line, the glass substrate or the focus line ofthe linear laser beam may be moved in the height direction of the glasssubstrate in accordance with a variation of the height of the convexsurface in the scanning direction.

According to the invention, there is provided a producing method of asemiconductor device, comprising, in irradiating and scanning anamorphous silicon film formed on a glass substrate with a linear laserbeam, the steps of: placing the glass substrate so that it assumes aninverted-U-shaped convex surface; irradiating and scanning, in a heatedstate, the amorphous silicon film with the linear laser beam having aninverted-U-shaped focus line that approximately coincides with theinverted-U-shaped convex surface; and performing cooling.

According to still another aspect of the invention, there is provided aproducing method of a semiconductor device, comprising, in converting anamorphous silicon film formed on a glass substrate into a crystallinesilicon film by heating and irradiating and scanning the crystallinesilicon film with a linear laser beam, the steps of: placing the glasssubstrate so that it assumes an inverted-U-shaped convex surface;irradiating and scanning, in a heated state, the crystalline siliconfilm with the linear laser beam having an inverted-U-shaped focus linethat approximately coincides with the inverted-U-shaped convex surface;and performing cooling.

In the above producing methods, the glass substrate may placed on astage having a convex surface or an inverted-U-shaped convex surfacewhile end portions of the glass substrate are pressed against the stage.

It is preferred that the heated state is such that the temperature ofthe glass substrate be kept in a range from a temperature higher thanthe room temperature to 70% of the strain absolute temperature of theglass substrate.

The heating of the glass substrate may be performed by heating a heliumgas with a heater that is provided under the glass substrate andcirculating the heated helium gas under the glass substrate.

In each of the above producing methods, it is preferred that the energydensity profile of the linear laser beam in its width direction satisfyinequalities 0.5L1<L2<L1 and 0.5L1<L3<L1 where L1 is a beam width at anenergy density that is 95% of a maximum energy density and L1+L2+L3 is abeam width at an energy density that is 70% of the maximum energydensity, L1 and L3 corresponding to both side portions of the energydensity profile. In particular, it is preferred that the linear laserbeam have a depth of focus that is about ±400 μm.

As described above, according to the invention, a flat glass substrateon which an amorphous silicon film is formed or a glass substrate thatis warped toward the film coating side after thermal crystallization ofthe amorphous silicon film is placed on the stage having a convexsurface or an inverted-U-shaped convex surface so as to conform to sucha surface. While this state is maintained and the glass substrate iskept at a particular temperature within a range from a temperaturehigher than the room temperature to 70% of the strain temperature(absolute temperature) of the glass substrate, a linear laser beamhaving a focus line that coincides with the curved surface of the glasssubstrate is applied to the amorphous or crystalline silicon film formedon the glass substrate. Thereafter, cooling is performed.

In the cooling that is performed after the laser light irradiation, thesilicon film contracts more than the glass substrate. As a result, theglass substrate is changed from the curved state to a flat state.

In the above manner, a crystalline silicon film having a uniformdistribution of crystallinity can be formed and the glass substrate canbe made flat. A crystalline silicon film having a uniform mobilitydistribution in the substrate surface can be obtained. Further, TFTshaving uniform characteristics can be obtained, and a liquid crystalelectro-optical device using those TFTs can be produced. Since the glasssubstrate is flat, the liquid crystal electro-optical device can beproduced easily with high accuracy.

The convex surface of the stage is so designed that a glass substratebearing a silicon film is rendered flat by the slow cooling that isperformed after the laser light irradiation.

To realize the above producing process, in performing laser annealing byusing a linear laser beam, the focus line of the irradiation laser beamis made to coincide with a shape of an irradiation surface (i.e., asectional shape in the beam longitudinal direction at a linear laserbeam irradiating position). Thus, uniform laser annealing is performedon a curved irradiation surface.

FIG. 1 shows an example of a linear laser beam 11 having aninverted-U-shaped focus line. If the irradiation surface has aninverted-U shape 12 as obtained when the substrate is curved in onedirection, a linear laser beam 11 having an inverted-U-shaped focus linethat conforms to the curved surface in its longitudinal direction isapplied to the irradiation surface through an optical system 10 so thatthe focus line is located on the irradiation surface, as shown in FIG.1.

FIG. 12 shows an example of a laser light irradiation method. As shownin FIG. 12, while a linear laser beam 21 having an inverted-U-shapedfocus line 20 is applied so that the focus line 20 is located on acurved surface of a glass substrate 23 that is placed on a stage 22, thestage 22 is moved relative to the laser beam 21 in its width direction(indicated by an arrow). Thus, laser annealing that is uniform in thesubstrate surface can be performed on the curved irradiation surface.

On the other hand, if an irradiation surface has not a simple inverted-Ushape but a convex shape in which the center of the surface is high andthe periphery is low, the irradiation surface has a height differencenot only in the longitudinal direction of a linear laser beam but alsoin its width direction (i.e., movement direction).

FIG. 13 shows another example of a laser light irradiation method. Inthis case, as a linear laser beam 31 having an inverted-U-shaped focusline is applied, a stage 33 on which a glass substrate 32 is placed ismoved in the height direction as well as in the horizontal direction(indicated by arrows) so that the irradiation surface always coincideswith the focus position (indicated by a broken line) of the laser beam31.

The position of the focus of the laser beam 31 may be controlled byadjusting the lens position while the height of the stage 33 on whichthe substrate 32 is placed is fixed.

The above control operations may be performed based on such data as aknown thickness of the substrate and the shape of a convex surface.Alternatively, the height of the substrate 32 or the focus line may bechanged automatically based on the height variation of the irradiationsurface which is measured with a laser displacement meter or the like.

To use a linear laser beam having an inverted-U-shaped focus line in itslongitudinal direction in performing laser annealing on a glasssubstrate that is forced to assume a convex surface, a cylindrical lensthrough which the linear laser beam passes immediately before strikingthe irradiation surface should have different focal lengths in thelongitudinal direction; for instance, it should have aninverted-U-shaped cross-section.

FIG. 9 shows a cylindrical lens 41 having different focal lengths in thelongitudinal direction, which is composed of a plurality of cylindricallenses 41a to 41e having different focal lengths.

FIG. 10 shows another cylindrical lens 42 having different focal lengthsin the longitudinal direction, which is constructed such that theplurality of cylindrical lenses 41a to 41e of FIG. 9 are smoothlyconnected to each other. The cylindrical lens 42 can provide a finerfocus line. The cylindrical lens 42 can also provide a variety of focusline shapes other than the inverted-U shape to accommodate varioussurface shapes of irradiation objects.

In addition to the above producing methods, it is effective to make alaser beam have the following energy density profile

FIGS. 15A and 15B show laser beam energy profiles. In the invention, alaser beam may have, at the focus, not only a conventional, ordinaryrectangular energy density profile shown in FIG. 15A in the widthdirection but also a trapezoidal one shown in FIG. 15B.

In the laser beam energy density profiles of FIGS. 15A and 15B, with anassumption that the maximum energy density of a laser beam is 1, a beamwidth at an energy density of 0.95 is represented by L1 and a beam widthat an energy density 0.75 is represented by L1+L2 and L3 where L2 and L3correspond to both side portions of the beam width.

According to the above definitions, a laser beam having a rectangularenergy density profile satisfies 0.5L1>L2 (or L3). L2 and L3 are notshown in FIG. 15A because they are very small.

Although a rectangular laser beam has a high energy density on theirradiation surface, its depth of focus is narrower than about ±200 μm.Where the irradiation surface has asperities or undulation, arectangular laser beam more likely cause a non-uniform distribution ofcrystallinity than a laser beam having the above-mentioned trapezoidalenergy density profile.

On the other hand, the energy density profile shown in FIG. 15Bsatisfies both inequalities 0.5L1<L2<L1 and 0.5L1<L3<L1 in the widthdirection of a linear laser beam.

A laser beam having the trapezoidal energy density profile can have adepth of focus that is narrower than about ±400 μm. Where theirradiation surface has asperities or undulation, a laser beam havingthe trapezoidal energy density profile provides a higher degree ofuniformity than a laser beam having the rectangular energy densityprofile. Further, a trapezoidal laser beam can provide an energy densitysufficient for crystallization.

Although a trapezoidal or triangular energy density profile where L2 (orL3)>L1 can provide a depth of focus wider than ±400 μm, it is associatedwith a problem of difficulty in focus adjustment. Further, since theenergy density is low, it is likely that the crystallization of asilicon film becomes insufficient, in which case a desired mobility maynot be obtained.

Thus, in irradiating a convex surface or an inverted-U-shaped convexsurface with a linear laser beam having an inverted-U-shaped focus line,a depth of focus wider than in the case of using a laser beam having theconventional rectangular energy density profile can be obtained byemploying a laser beam having the trapezoidal energy density profile inthe width direction which satisfies the above inequalities.

FIGS. 16A and 16B show an example of a laser light irradiation methodwith a linear laser beam having the trapezoidal energy density profile.

In FIG. 16A, by giving the trapezoidal energy density profile to alinear laser beam 51 having an inverted-U-shaped focus line, a siliconfilm formed on a substrate 53 can be crystallized sufficiently anduniformly with only horizontal movement of a stage 52 if the heightdifference due to asperities, undulation, etc. of the irradiationsurface of the substrate 53 falls within the range of ±400 μm. That is,it is not necessary to move the substrate 53 in the height direction,contributing to simplification of the apparatus and reduction of thecost.

If a laser beam having the trapezoidal energy density profile is used inaddition to moving the substrate in the height direction, the focusingmargin in the height direction can be increased from the conventionalcase. Naturally the focusing margin with respect to a height differenceof the irradiation surface in the longitudinal direction of a linearlaser beam can be increased.

Where the irradiation surface is a convex surface that is not a clearinverted-U shape, the shape of the irradiation surface may not bestraight in the substrate movement direction. Therefore, there is apossibility that even if a laser beam has an inverted-U-shaped focusline, it comes out of focus with the irradiation surface.

To solve this problem, a laser beam having a trapezoidal energy densityprofile and a wide depth of focus is used. As a result, as shown in FIG.16B, with an inverted-U-shaped focus line 62 of a laser beam 61, themargin is increased as much as an increase in the depth of focus 63, sothat the uniformity in the longitudinal direction (horizontal directionon the paper surface of FIG. 16B) can be improved.

When a linear laser beam having a trapezoidal energy density profile ismoved relative to the substrate in the width direction of the laser beam(i.e., perpendicularly to its longitudinal direction) with overlaps ofbeams, due to gradients in the energy density profile of the laser beam,an arbitrary point on the irradiation surface is first irradiated withweak laser beams. Thereafter, at that point, the intensity of laserbeams gradually increases, and then gradually decreases, thus completingthe irradiating operation.

FIG. 17 shows how laser light irradiation is performed with a linearlaser beam having a trapezoidal energy density profile. The laser beamenergy density gradually increases in the head portion of the laserlight irradiation (indicated by A in FIG. 17) and gradually decreases inthe end portion (indicated by B in FIG. 17). Thus, when a linear laserbeam having the above trapezoidal energy density profile is used, thevariation in the density of energy supplied to the irradiation areabecomes much gentler than in the case of using a linear laser beamhaving the conventional rectangular energy density profile.

This type of laser light irradiation can provide effects equivalent tothose obtained by the conventional two-step irradiation in whichlow-energy-density laser light is applied first (preliminaryirradiation) and then high-energy-density laser light is applied (mainirradiation).

As a result, abrupt phase changes can be prevented from occurring in alaser-light-irradiated amorphous silicon film, whereby surfaceroughening and accumulation of internal stress can be prevented. Thus, auniform distribution of crystallinity can be attained.

In the invention, a substrate heating method as shown in FIG. 14 enablesefficient heating during the laser light irradiation. That is, in astate that a substrate 71 is fixed to a stage 73 by means of holdingmembers 72, a helium gas 75 is heated with a heater 74 that is installedunder the stage 73 and the heated helium gas 75 is circulated under thesubstrate 71 by a pump 76, whereby the substrate 71 can be kept at adesired temperature. The reason for using the helium gas 75 is its largeheat conductivity.

The inventors checked influences on the substrate shape of everyproducing step for forming TFTs on a substrate, and found that substratedeformation due to a heat treatment for crystallizing a silicon film wasmost remarkable. No marked deformation was found after that step.

Therefore, if a substrate is very flat after thermal crystallization orlaser light irradiation in a heated state, the substrate can be kept ina fairly flat state even after completion of the entire process. Forthis reason, by performing laser annealing on a silicon film that isformed on a glass substrate according to the method of the invention, acrystalline silicon film that is flat and has a highly uniformdistribution of crystallinity in the substrate surface.

By forming a number of TFTs by using the crystalline silicon film, thedistribution of threshold voltages of the TFTs can be made very uniformin the substrate surface. This effect becomes more remarkable as thesubstrate area increases.

Where a glass substrate has a size of about 100 mm×100 mm and athickness of about 1 mm, the convex surface of the stage is such thatthe height difference of the convex surface between a central portionand end portions (lowest portions) in the area covered by glasssubstrate is 20 to 200 μm, for instance, about 50 μm.

Where a glass substrate has a size of about 500 mm×500 mm (for instance,370 mm×400 mm, 400 mm×500 mm, or 550 mm×650 mm) and a thickness of about0.5 to 0.7 mm, the convex surface of the stage is such that the heightdifference of the convex surface between a central portion and endportions (lowest portions) in the area covered by glass substrate isabout 1 to 2 mm.

If a liquid crystal display is formed by using a glass substrate onwhich crystalline silicon TFTs for pixels or driving have been formedaccording to the method of the invention, the cell assembling can beperformed easily and positively by virtue of the glass substrate havinga very high degree of flatness. In this case, the flattening of asubstrate, which is the advantageous effect of the invention, is stilleffective even if there is no crystallization step with laser lightirradiation after the thermal crystallization.

According to the invention, where a glass substrate has a size of 100mm×100 mm and a thickness of 1.1 mm, the height difference due tosurface roughening and undulation of the glass substrate can be madeless than about 10 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a linear laser beam having aninverted-U-shaped focus line;

FIG. 2 shows a distribution, in the substrate surface, of thresholdvoltages of TFTs formed by using a crystalline silicon film according toa conventional method;

FIG. 3 shows how laser annealing is performed with linear laser light ona silicon substrate formed on a glass substrate having a warp.

FIG. 4 shows an arrangement of TFTs on a glass substrate;

FIG. 5 shows a distribution, in the substrate surface, of thresholdvoltages of TFTs formed by using a crystalline silicon film that wasformed according to a first embodiment of the present invention;

FIG. 6 shows the concept of a laser annealing apparatus used inembodiments of the invention;

FIGS. 7A to 7C and 8A and 8B show examples of optical systems used inthe embodiments of the invention;

FIGS. 9 and 10 show examples of cylindrical lenses having differentfocal lengths in the longitudinal direction;

FIGS. 11A-11F show producing processes according to the embodiments ofthe invention;

FIGS. 12 and 13A to 13C show examples of laser light irradiationmethods;

FIG. 14 shows an example of a substrate heating method;

FIGS. 15A and 15B show laser beam energy density profiles;

FIGS. 16A and 16B show an example of a laser light irradiation method ina case where a linear laser beam has a trapezoidal energy densityprofile; and

FIG. 17 shows how laser light irradiation is performed with a linearlaser beam having a trapezoidal energy density profile.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

This embodiment is directed to a case where laser annealing is performedafter an amorphous silicon film formed on a glass substrate is thermallycrystallized and TFTs are formed by using a film thus formed.

FIGS. 11A to 11F show a producing process according to this embodiment.

A glass substrate 101 is a 100 mm×100 mm, 0.7-mm-thick Corning 1737glass substrate. Corning 7059, OA2, and NA45 glass substrates and thelike may also be used. First, a 2,000-Å-thick under silicon oxide film102 is formed on the glass substrate 101 by plasma CVD and, immediatelythereafter, a 500-Å-thick amorphous silicon film 103 is formed thereonalso by plasma CVD. After a 10-ppm nickel acetate aqueous solution isapplied to the surface of the amorphous silicon film 103, a nickelacetate layer is formed by spin coating. Better results were obtained byadding a surfactant to the nickel acetate aqueous solution. Since thenickel acetate layer is very thin, it does not always assume a film. Butthis does not cause any problems in the subsequent steps. (FIG. 11A)

The amorphous silicon film 103 is crystallized by thermal annealing at550° C. for 4 hours. In this step, nickel serves as nuclei ofcrystallization, thus accelerating crystallization of the amorphoussilicon film 103. The Corning 1737 glass substrate has a straintemperature of 667° C., which is lower than the annealing temperature of550° C.

When the glass substrate 101 is slowly cooled after the above thermalcrystallization, the silicon film contracts and a concave warp occurs inthe substrate 101.

It is due to the function of nickel that the thermal annealing can beperformed at a low temperature (550° C.; lower than the straintemperature of Corning 1737) and in a short time (4 hours). Detailed aredescribed in Japanese Unexamined Patent Publication No. 6-244104. Thispublication states that thermal annealing be performed, for instance, at550° C. for 4 hours so that the thermal annealing temperature does notexceed the strain temperature of a glass substrate. This temperature wasdetermined to prevent large deformation of a glass substrate.

Better results were obtained when the concentration of the catalystelement was 1×10¹⁵ to 1×10¹⁹ atoms/cm³. When the concentration washigher than 1×10¹⁹ atoms/cm³, metal properties appeared in silicon, thatis, semiconductor characteristics disappeared. In this embodiment, theconcentration of the catalyst element in the silicon film was 1×10¹⁷ to5×10¹⁸ atoms/cm³ in terms of a minimum value in the film. That is, thesevalues are minimum values of the concentration of the catalyst elementin silicon films as analyzed and measured by secondary ion massspectrometry (SIMS).

To correct the warp in the glass substrate 101 occurring after the abovethermal crystallization step and to have the crystallization furtherproceed, the glass substrate 23 is forced to assume an inverted-U shapeby placing the glass substrate 23 on a stage 22 having aninverted-U-shaped convex surface (see FIG. 12) and bringing the glasssubstrate 23 into close contact with the stage 22 by proper holdingmembers 24.

In this state, to improve the crystallinity of the crystalline siliconfilm, a laser beam emitted from an excimer laser, which is alarge-output pulsed laser, is applied thereto.

A description will be made of a laser annealing apparatus used in thisembodiment.

FIG. 6 shows the laser annealing apparatus, used in this embodiment, ofa multi-chamber type. A substrate is placed through a loader/unloaderchamber 201, and then properly positioned in an alignment chamber 202.The substrate is consecutively transferred to the respective chambersthrough a transfer chamber 203 by a substrate transfer robot 204provided in the transfer chamber 203, and processed in those chambers.

A substrate is transferred to a heat treatment chamber 205, where it issubjected to a heat treatment such as preliminary heating. Then, thesubstrate is subjected to laser annealing in a laser annealing chamber206, slowly cooled in a cooling chamber 207, and moved to aloader/unloader chamber 201.

In the laser annealing apparatus concerned, the energy variation amonglaser pulses falls within ±3% in terms of 3σ. Although a pulsed laserhaving a larger energy variation than the above range may be used, thedepth of focus is narrow in such a case. A pulsed laser whose energyvariation is larger than ±10% in terms of 3σ is not suitable for use inthis embodiment.

Type EX748 produced by Lumnics Corp. is used as an oscillator. Thisoscillator emits KrF excimer laser light (wavelength: 248 nm; pulsewidth: 25 ns). Naturally other excimer lasers and other types of lasersmay be used as long as they are of a pulsed oscillation type.

Being airtight, this laser annealing apparatus is free of impuritycontamination. This laser annealing apparatus has an atmosphere controlfunction. Further, having a substrate heating function, this laserannealing apparatus can keep an irradiation object at a desiredtemperature during laser light irradiation.

To deform the shape of an emitted laser beam, it is introduced into anoptical system as shown in FIG. 7.

A laser beam assuming a rectangle of about 3 cm×2 cm immediately beforeentering the optical system is shaped by the optical system into a longand narrow beam (i.e., linear beam) of 10 to 30 cm in length and 0.01 to0.3 cm in width.

After passing through the optical system, the linear laser beam has afocus distribution as shown in FIG. 1, and assumes a trapezoidal energydensity profile as shown in FIG. 15B in the width direction. Afterpassing through the optical system, the linear laser beam has a maximumenergy of 800 mJ/shot.

The reason for shaping an emitted laser beam into a long and narrow beamis to improve its workability. That is, if a linear laser beam is longerthan the width of a sample, the entire sample can be irradiated bymoving the sample in one direction. Even if the linear laser beam isshorter than the width of a sample, the laser processing is completed ina shorter time than in the case of using a rectangular beam. However, inthis case, the beam needs to be moved relative to the sample in the twoorthogonal directions.

A stage of a substrate (i.e., sample) to be irradiated with a laser beamis controlled by a computer, and is so designed as to be movableperpendicularly to the longitudinal direction of the linear laser beam.If the stage is further provided with a function of moving it in thelongitudinal direction of a linear beam, the entire sample can besubjected to laser processing even if the beam is shorter than the widthof a sample.

The stage 73 has a structure shown in FIG. 14. A sample can be kept at agiven temperature during laser light irradiation while being heated bymeans of a He gas 75.

Referring to FIGS. 7A and 7B, a description will be made of an opticalpath in the optical system for shaping a laser beam into a linear beam.

An incident laser beam passes through a cylindrical concave lens B, acylindrical convex lens C (the lenses B and C are together called a beamexpander), and fly-eye lenses D and D2.

The laser beam is then passed through a cylindrical convex lens E (firstcylindrical lens) and a cylindrical convex lens F (second cylindricallens) that is provided to improve the uniformity of the beam in thelongitudinal direction, reflected by a mirror G, converged by acylindrical lens H, and finally applied to the irradiation surface.

The distances (each being the sum of the focal lengths of the respectivelenses concerned) between the cylindrical lenses A and B, between thefly-eye lenses D and D2, between the fly-eye lens D and the cylindricallens E, and between the cylindrical lens F and the irradiation surfaceare set at 230 mm, 230 mm, 650 mm, and 650 mm, respectively. Apparentlythese values can be changed as occasion demands.

The cylindrical lens H has a focal length of 120 mm. In a structure asshown in FIG. 9 or 10, the cylindrical lens H forms an inverted-U-shapedfocus line on the irradiation surface.

Any optical system may be employed as long as it can shape a laser beaminto a linear beam as required in the invention. For example, it iseffective to use an optical system shown in FIG. 8 which is not providedwith the lenses B and C.

A laser beam is shaped into a linear beam so as to have a beam area of125 mm×1 mm at the irradiation position. The width of a laser beam isdefined as a half width of the maximum value of its energy density.

The shape of the energy density profile of a laser beam at the focus isadjusted to be trapezoidal by moving the lens H vertically (in directionJ) (FIG. 7C). Alternatively, the energy density profile of a laser beamon the irradiation surface (i.e, at the focus) can be changed from ashape close to a rectangle to a shape close to a trapezoid by moving theirradiation surface vertically (in direction J) with respect to the lensH.

Referring to FIG. 15B, the energy density profile in the width directionof a linear laser beam is trapezoidal in which L1=0.4 mm and L2=L3=0.25mm, which satisfy inequalities 0.5L1<L2<L1 and 0.5L1<L3<L1. In thiscase, the depth of focus is about ±400 mm.

As for the laser light irradiation method, as shown in FIG. 16A,irradiation is performed such that a linear laser beam 51 is movedrelative to an irradiation object by moving a stage 52 having a convexsurface on which a glass substrate 53 is placed only horizontally (indirection I in FIG. 7B).

The linear beam 51 is relatively moved approximately perpendicularly toits longitudinal direction so that an inverted-U-shaped focus line ofthe laser beam is always located in the surface of the glass substrate53.

The irradiation surface of the substrate 53 is a convex surface thatconforms to the shape of the stage 52, and has a height difference ofabout 300 μm also in the movement direction of the substrate 53.

As shown in FIGS. 13A to 13C, the laser light irradiation may beperformed while the height (in direction J in FIG. 7B) of a stage 33 onwhich a substrate 32 is placed is also changed in accordance with theheight difference of the convex surface.

The degree of expanse of the bottom portion of the above trapezoidalprofile varies with the distance between the final lens of the opticalsystem and the irradiation surface. During the laser processing, thedistance between the final lens of the optical system and theirradiation surface varies due to asperities of an irradiation object,and the degree of expanse of the bottom portion of the laser beamtrapezoidal profile varies accordingly. However, if the variation rangeis such that the above inequalities are satisfied, uniform laserprocessing is assured. The term "uniform" as used herein means that amobility variation in the substrate surface of a film that has beenirradiated with laser light falls within ±10%.

A sample is placed on the stage, and the irradiation is performed whilethe stage is moved at 2 mm/s. As for the laser light irradiationconditions, the laser light energy density is 100 to 500 mJ/cm² (300mJ/cm² in this embodiment) and the pulse rate is 30 pulses/s. The term"energy density" as used herein means the density of the top baseportion (having a maximum value) of a trapezoidal beam profile.

If the laser light irradiation is performed under the above conditions,a certain point of the sample is subjected to 15-step irradiation. Thatis, since pulse beams take 0.5 second to pass one point of the sample,the one point is irradiated with 15 pulse beams in each scan. Of the 15times of irradiation, the irradiation energy density gradually increasesin first several times of irradiation and gradually decreases in lastseveral times of irradiation. FIG. 17 shows how the irradiation iseffected. The laser beam energy density gradually increases in the headportion of the 15 steps (indicated by A) and gradually decreases in theend portion (indicated by B).

This type of laser light irradiation can provide superior crystallinitywith low degrees of surface roughness and accumulation of internalstress which crystallinity is equivalent to that obtained by theconventional two-step irradiation, without using a plurality of laserbeams having different energy densities.

According to an experiment by the inventors, 3 to 100 steps ofirradiation, preferably 10 to 40 steps of irradiation, provided siliconfilms with the best crystallinity.

During the laser light irradiation, the substrate temperature is from atemperature higher than the room temperature to about 70% of the straintemperature of the glass substrate. In this embodiment, it is 200° C.(FIG. 11B)

When the glass substrate is slowly cooled after completion of the abovestep, the silicon film that is formed on the substrate contracts morethan the substrate. As a result, the glass substrate assumes a very highdegree of flatness.

Then, TFTs as semiconductor devices are formed by using the crystallinesilicon film obtained above. The TFTs are formed on the substrate inmatrix form. Specifically, 400×300 TFTs are formed in the area of 40mm×50 mm. A producing process will be described below.

The silicon film is etched into an island-like silicon region 105. A1,200-Å-thick silicon oxide film 106 as a gate insulating film is thendeposited by plasma CVD by using material gases of TEOS and oxygen.During the film deposition, the substrate temperature is kept at 250° to380° C., for example, 300° C. (FIG. 11C)

Subsequently, an aluminum film (containing silicon at 0.1 to 2%) havinga thickness of 3,000 to 8,000 Å, for example, 6,000 Å, is deposited bysputtering. A gate electrode 107 is formed by etching the aluminum film.(FIG. 11C)

An impurity (boron) is implanted into the silicon region 105 by iondoping with the gate electrode 107 used as a mask. Diborane (B₂ H₂)diluted with hydrogen to 1 to 10%, for instance, 5%, is used as a dopinggas. The acceleration voltage is 60 to 90 kV, for instance, 65 kV, andthe dose is 2×10¹⁵ to 5×10¹⁵ atoms/cm², for instance, 3×10¹⁵ atoms/cm².During the ion doping, the substrate temperature is the roomtemperature. As a result, P-type impurity regions 108 (source) and 109(drain) are formed. (FIG. 11D)

To activate boron thus implanted, optical annealing is performed byusing KrF excimer laser light. The energy density of laser light is 100to 350 mJ/cm², for instance, 250 mJ/cm². Crystallinity is improved bypreliminary irradiation at an energy density of about 170 mJ/cm² beforethe above irradiation. As for the laser light irradiation method, alinear laser beam is applied to the irradiation object while the formeris moved relative to the latter approximately perpendicularly to thelongitudinal direction of the linear laser beam. Setting is so made thatan arbitrary point of the irradiation object is irradiated with 2 to 20shots of laser beams. During the laser light irradiation, the substratetemperature is 200° C. Thereafter, thermal annealing is performed at450° C. for 2 hours in a nitrogen atmosphere. (FIG. 11E)

A 6,000-Å-thick silicon oxide film 110 as an interlayer insulating filmis formed by plasma CVD, and contact holes are formed through it.Source/drain electrodes/wiring lines 111 and 112 of the TFT are formedwith a metal film, for instance, a multi-layer film of titanium andaluminum. Thermal annealing is performed at 200° to 350° C. in ahydrogen atmosphere of 1 atm. (FIG. 11F)

FIG. 5 shows a distribution, in the substrate surface, of thresholdvoltages of TFTs using the crystalline silicon film formed according tothis embodiment. In FIG. 5, as in the case of FIG. 2, the horizontalaxis represents the positions of the respective TFTs shown in FIG. 4(enclosed by a broken line).

As seen from FIG. 5, the TFTs that were produced according to thisembodiment have a uniform distribution of threshold voltage in thesubstrate surface. It is apparent that the distribution of FIG. 5 ishigher in the degree of uniformity than the conventional distribution ofFIG. 2.

In this embodiment, the irradiation object has a convex surface with aheight difference of about 300 μm. Although the substrate is moved onlyin the horizontal direction, the depth of focus of a laser beam is about±400 μm. Therefore, the mobility variation, in the substrate surface, ofthe crystallized coating was about ±8%, which means that the laserprocessing was conducted uniformly.

Another experiment was conducted in which the energy density profile ofa linear laser beam in the width direction was set such that L1=0.5 mmand L2=L3=0.2 mm, which satisfy 0.5L1>L2=L3 (see FIG. 15B) and means aprofile somewhat closer to a rectangle, and laser annealing wasperformed on a similar irradiation object (i.e., silicon film). In thiscase, the mobility variation was ±15%.

A further experiment was conducted in which the energy density profileof a linear laser beam in the width direction was set such that L1=0.2mm and L2=L3=0.3 mm, which satisfy L1<L2=L3 (see FIG. 15B) and means aprofile somewhat closer to a triangle, and laser annealing was performedon a similar irradiation object (i.e., silicon film). In this case,although the mobility variation was ±8%, the mobility values were verysmall for a crystalline silicon film.

Embodiment 2

This embodiment is directed to a case where an amorphous silicon filmformed on a glass substrate is crystallized by laser annealing and TFTsare formed by using a film thus formed.

A producing process according to this embodiment will be described withreference to FIGS. 11A to 11F, which were also used above in thedescription of the first embodiment.

A glass substrate 101 is a 100 mm×100 mm, 0.7-mm-thick Corning 1737glass substrate. Corning 7059, OA2, and NA45 glass substrates and thelike may also be used. First, a 2,000-Å-thick undercoat silicon oxidefilm 102 is formed on the glass substrate 101 by plasma CVD and,immediately thereafter, a 500-Å-thick amorphous silicon film 103 isformed thereon also by plasma CVD.

The following is performed not only to crystallize the amorphous siliconfilm 103 formed on the glass substrate 101 but also to suppresssubstrate deformation by the crystallization.

The glass substrate 71 is placed on a stage 73 having aninverted-U-shaped convex surface (see FIG. 14) and brought into closecontact with the stage 73 by proper holding members 72. Further, theglass substrate 71 is heated to a temperature within the range from atemperature higher than the room temperature to about 70% of the straintemperature of the glass substrate 71. In this embodiment, the glasssubstrate 71 is heated to 200° C.

As for the heating method, the heating can be performed efficiently byusing a helium gas as shown in FIG. 14.

To crystallize the amorphous silicon film 103, laser annealing isperformed by irradiating it with laser light emitted from an excimerlaser, which is a large-output pulsed laser. As in the case of the firstembodiment, the laser annealing apparatus of FIG. 6 is used in thisembodiment.

In this embodiment, type 3000-308 produced by Lambda Physic Corp. isused as an oscillator. This oscillator emits XeCl excimer laser light(wavelength: 308 nm; pulse width: 26 nsec).

To deform the shape of an emitted laser beam, it is introduced into anoptical system as shown in FIG. 8.

A laser beam assuming a rectangle of about 3 cm×2 cm immediately beforeentering the optical system is shaped by the optical system into a longand narrow beam (i.e., linear beam) of 10 to 30 cm in length and 0.01 to0.3 cm in width.

After passing through the optical system, the linear laser beam assumesa rectangular energy density profile as shown in FIG. 15A in the widthdirection, and has a focus distribution as shown in FIG. 1 in thelongitudinal direction. After passing through the optical system, thelinear laser beam has a maximum energy of 1,000 mJ/shot.

Referring to FIGS. 8A and 8B, a description will be made of an opticalpath in the optical system for shaping a laser beam into a linear beam.

A laser beam emitted from a laser light source a and input to theoptical system passes through fly-eye lenses b and c.

The laser beam is then passed through a cylindrical convex lens d (firstcylindrical lens) and a cylindrical convex lens e (second cylindricallens) that is provided to improve the uniformity of the beam in thelongitudinal direction, reflected by a mirror f, converged by acylindrical lens g, and finally applied to the sample.

As for the optical path lengths, the distance between the laser lightsource a to the mirror g is 2,000 mm and the distance between the mirrorf and the irradiation surface is 440 mm. The cylindrical lens g has afocal length of about 100 mm.

A laser beam is shaped into a linear beam so as to have a beam area of300 mm×0.4 mm at the irradiation position. The width of a laser beam isdefined as a half width of its irradiation energy density.

As for the laser light irradiation method, the irradiation is performedwhile the linear laser beam is moved relative to the irradiation objectas shown in FIG. 12. The linear laser beam (or the substrate) is movedapproximately perpendicularly to its longitudinal direction (direction hin FIG. 8B).

The laser light irradiation is performed such that an inverted-U-shapedfocus line of the laser beam is always located in the curved surface ofthe glass substrate surface. The substrate is so curved as to assume aclear inverted-U shape, and the height difference in the substratemovement direction is much smaller than in the curving direction(longitudinal direction of the laser beam).

The energy density of laser light is 100 to 500 mJ/cm², for instance,370 mJ/cm². Crystallinity is improved by preliminary irradiation at anenergy density of about 220 mJ/cm² before the above irradiation. Anarbitrary point on the irradiation surface is irradiated with 2 to 20shots of laser beams.

During the laser light irradiation, the substrate temperature is from atemperature higher than the room temperature to about 70% of the straintemperature of the glass substrate. In this embodiment, the substratetemperature is 200° C. (FIG. 11B)

When the glass substrate is slowly cooled after completion of the abovestep, the silicon film that is formed on the substrate contracts morethan the substrate. As a result, the glass substrate assumes a very highdegree of flatness.

Thereafter, TFTs are formed in the same manner as in the firstembodiment.

As in the case of the first embodiment, the distribution, in thesubstrate surface, of threshold voltages of the TFTs thus produced wasmuch higher in the degree of uniformity than that of TFTs that wereproduced without flattening a glass substrate.

Embodiment 3

As in the case of the second embodiment, this embodiment is directed toa case where an amorphous silicon film formed on a glass substrate iscrystallized by laser annealing and TFTs are formed by using a film thusformed, and the glass substrate is placed on a stage having a convexsurface.

A producing process according to this embodiment will be described withreference to FIGS. 11A-11F, which were also used above in thedescription of the first embodiment.

A glass substrate 101 is a 100 mm×100 mm, 0.7-mm-thick Corning 1737glass substrate. Corning 7059, OA2, and NA45 glass substrates and thelike may also be used. A 2,000-Å-thick under silicon oxide film 102 isformed on the glass substrate 101 by plasma CVD and, immediatelythereafter, a 500-Å-thick amorphous silicon film 103 is formed thereonalso by plasma CVD.

The following step is performed not only to crystallize the amorphoussilicon film 103 formed on the glass substrate 101 but also to suppresssubstrate deformation by the crystallization.

The glass substrate 101 is placed on a stage having a convex surface andbrought into close contact with the stage by proper holding members. Theglass substrate 101 is heated at a temperature range from a temperaturehigher than the room temperature to about 70% of the strain temperatureof the glass substrate 101. In this embodiment, the glass substrate 101is heated to 200° C.

As for the heating method, the heating can be performed efficiently byusing a helium gas as shown in FIG. 14.

To crystallize the amorphous silicon film 103, laser annealing isperformed by irradiating it with laser light emitted from an excimerlaser, which is a large-output pulsed laser. As in the case of the firstembodiment, the laser annealing apparatus of FIG. 6 is used in thisembodiment.

In this embodiment, the oscillator, the optical system, the laser beamshape, and the focus line are the same as those in the secondembodiment.

The energy density profile of a linear laser beam in the width directionis rectangular as shown in FIG. 15A.

As for the laser light irradiation method, the irradiation is performedwhile the linear laser beam is moved relative to the irradiation objectas shown in FIG. 13. The linear laser beam (or the substrate) is movedapproximately perpendicularly to its longitudinal direction (direction hin FIG. 8B).

The glass substrate 101 has a height difference also in the substratemovement direction. However, since the energy density profile of thelinear laser beam in the width direction is rectangular, the depth offocus is not wide. In view of this, the stage on which the substrate 101is placed is moved in the height direction (direction i in FIG. 8B) asshown in parts FIGS. 13A to 13C so that the inverted-U-shaped focus lineof the laser beam is located in the surface of the glass substrate 101.

The energy density of laser light is 100 to 500 mJ/cm², for instance,370 mJ/cm². Crystallinity is improved by preliminary irradiation at anenergy density of about 220 mJ/cm² before the above irradiation. Anarbitrary point of the irradiation object is irradiated with 2 to 20shots of laser beams.

During the laser light irradiation, the substrate temperature is from atemperature higher than the room temperature to about 70% of the straintemperature of the glass substrate. In this embodiment, the substratetemperature is 200° C. (FIG. 11B)

When the glass substrate is slowly cooled after completion of the abovestep, the silicon film that is formed on the substrate contracts morethan the substrate. As a result, the glass substrate assumes a very highdegree of flatness.

Thereafter, TFTs are formed in the same manner as in the firstembodiment.

As in the case of the first embodiment, the distribution, in thesubstrate surface, of threshold voltages of the TFTs thus produced wasmuch higher in the degree of uniformity than that of TFTs that wereproduced without flattening a glass substrate.

According to the invention, a glass substrate on which a crystallinesilicon film is formed is flattened, whereby a crystalline silicon filmwhose crystallinity is high and uniform in the substrate surface can beobtained even after a laser light irradiation step.

By using such a crystalline silicon film, crystalline silicon TFTshaving a uniform distribution of threshold voltages in the substratesurface can be produced.

The invention is particularly advantageous in forming a large number ofTFTs on a large-area glass substrate.

In producing a liquid crystal display by using a glass substrateaccording to the invention, the cell assembling can be performed easilyand positively because the substrate is flat. As such, the invention isvery useful from the industrial viewpoint.

What is claimed is:
 1. A method for producing a semiconductor devicecomprising the step of irradiating a linear laser beam into an unevensurface on which a semiconductor film is formed, wherein the linearlaser beam has a focus line that extends in a longitudinal direction ofthe linear laser beam and approximately coincides with a sectional shapeof the surface.
 2. A method for producing a semiconductor device,comprising the step of irradiating and scanning a semiconductor filmformed on an uneven surface with a linear laser beam, wherein the linearlaser beam has a focus line that extends in a longitudinal directionthereof and approximately coincides with a sectional shape of thesurface.
 3. A method for producing a semiconductor device,comprising:forming a semiconductor layer comprising amorphous silicon ona glass substrate; placing the glass substrate so as to have a convexsurface; irradiating and scanning, in a heated state, the semiconductorlayer with the linear laser beam having an inverted-U-shaped focus linethat approximately coincides with the convex surface; and then coolingthe semiconductor layer.
 4. A method for producing a semiconductordevice, comprising:heating a semiconductor layer comprising amorphoussilicon formed on a glass substrate to obtain a crystallinesemiconductor layer comprising silicon; placing the glass substrate soas to have a convex surface; irradiating and scanning, in a heatedstate, the crystalline semiconductor layer with the linear laser beamhaving an inverted-U-shaped focus line that approximately coincides withthe convex surface; and then cooling the semiconductor layer.
 5. Amethod for producing a semiconductor device, comprising:forming anamorphous semiconductor layer comprising silicon on a glass substrate;placing the glass substrate so as to have a convex surface; irradiatingand scanning, in a heated state, a surface of the amorphoussemiconductor layer with the linear laser beam having aninverted-U-shaped focus line that approximately coincides with theconvex surface while moving the glass substrate in a height directionthereof so that the irradiated surface always coincides with a focusposition of the linear laser beam; and then cooling the semiconductorlayer.
 6. A method for producing a semiconductor device, comprising thesteps of:heating an amorphous semiconductor layer comprising siliconformed on a glass substrate to obtain a crystalline semiconductor layercomprising silicon; placing the glass substrate so as to have a convexsurface; irradiating and scanning, in a heated state, a surface of thecrystalline semiconductor layer with the linear laser beam having aninverted-U-shaped focus line that approximately coincides with theconvex surface while moving the glass substrate in a height directionthereof so that the irradiated surface always coincides with a focusposition of the linear laser beam; and then cooling the semiconductorlayer.
 7. A method for producing a semiconductor device,comprising:forming an amorphous semiconductor layer comprising siliconon a glass substrate; placing the glass substrate so as to have a convexsurface; irradiating and scanning, in a heated state, a surface of theamorphous semiconductor layer with the linear laser beam having aninverted-U-shaped focus line that approximately coincides with theconvex surface while moving the focus line of the linear laser beam in aheight direction of the glass substrate so that the irradiated surfacealways coincides with a focus position of the linear laser beam; andthen cooling the semiconductor layer.
 8. A method for producing asemiconductor device, comprising:heating an amorphous semiconductorlayer comprising silicon formed on a glass substrate to obtain acrystalline semiconductor layer comprising silicon; placing the glasssubstrate so as to have a convex surface; irradiating and scanning, in aheated state, a surface of the crystalline semiconductor layer with thelinear laser beam having an inverted-U-shaped focus line thatapproximately coincides with the convex surface while moving the focusline of the linear laser beam in a height direction of the glasssubstrate so that the irradiated surface always coincides with a focusposition of the linear laser beam; and then cooling the semiconductorlayer.
 9. The method of claims 3 wherein the glass substrate is placedon a stage having a convex surface while end portions of the glasssubstrate are pressed against the stage.
 10. The method of claims 4wherein the glass substrate is placed on a stage having a convex surfacewhile end portions of the glass substrate are pressed against the stage.11. The method of claims 5 wherein the glass substrate is placed on astage having a convex surface while end portions of the glass substrateare pressed against the stage.
 12. The method of claims 6 wherein theglass substrate is placed on a stage having a convex surface while endportions of the glass substrate are pressed against the stage.
 13. Themethod of claims 7 wherein the glass substrate is placed on a stagehaving a convex surface while end portions of the glass substrate arepressed against the stage.
 14. The method of claims 8 wherein the glasssubstrate is placed on a stage having a convex surface while endportions of the glass substrate are pressed against the stage.
 15. Amethod for producing a semiconductor device, comprising:forming anamorphous semiconductor layer comprising silicon on a glass substrate;placing the glass substrate so as to have an inverted-U-shaped convexsurface; irradiating and scanning, in a heated state, the amorphoussemiconductor layer with the linear laser beam having aninverted-U-shaped focus line that approximately coincides with theinverted-U-shaped convex surface; and then cooling the semiconductorlayer.
 16. A method for producing a semiconductor device,comprising:heating an amorphous semiconductor layer comprising siliconformed on a glass substrate to obtain a crystalline semiconductor layercomprising silicon; placing the glass substrate so as to have aninverted-U-shaped convex surface; irradiating and scanning, in a heatedstate, the crystalline semiconductor layer comprising silicon with thelinear laser beam having a inverted-U-shaped focus line thatapproximately coincides with the inverted-U-shaped convex surface; andthen cooling the semiconductor layer.
 17. The method of claim 15 whereinthe glass substrate is placed on a stage having an inverted-U-shapedconvex surface while end portions of the glass substrate are pressedagainst the stage.
 18. The method of claim 16 wherein the glasssubstrate is placed on a stage having an inverted-U-shaped convexsurface while end portions of the glass substrate are pressed againstthe stage.
 19. A liquid crystal electro-optical device producing methodcomprising:forming an amorphous semiconductor layer comprising siliconon a glass substrate; placing the glass substrate so as to have a convexsurface; irradiating and scanning, in a heat state, the amorphoussemiconductor layer comprising silicon with the linear laser beam havingan inverted-U-shaped focus line that approximately coincides with theconvex surface; cooling the semiconductor layer after the irradiatingand scanning step; forming thin film transistors by using the cooledsemiconductor layer; and producing a liquid crystal electro-opticaldevice by using the glass substrate on which the thin film transistorshave been formed.
 20. A method for producing a semiconductor device,comprising:forming an semiconductor layer comprising silicon on a glasssubstrate; generating a pulse laser light in a laser oscillator; shapingsaid pulse laser light into a linear pulse laser light having a focusline; and irradiating said linear pulse laser light to saidsemiconductor layer while moving said linear pulse laser lightrelatively with respect to said semiconductor layer comprising siliconapproximately perpendicularly to longitudinal direction of said linearpulse laser light, to crystallize said semiconductor layer, wherein saidpulse laser light has an energy variation falling within ±3% in terms of3 σ among laser pulses in said oscillator; and wherein said focus lineapproximately coincides with an irregular surface of said glasssubstrate.
 21. The method of claim 20 wherein said pulse laser light hasa wavelength of 248 nm.
 22. The method of claim 20 further comprisingcrystallizing said semiconductor layer comprising silicon,whereincrystallinity of the crystallized semiconductor layer is improved bysaid irradiating.
 23. The method of claim 20 wherein said glasssubstrate has a temperature from a temperature higher than roomtemperature to about 70% of a strain temperature of said glass substratein said irradiating.
 24. The method of claim 23 wherein said straintemperature is in view of absolute temperature.
 25. The method of claim20 wherein said linear pulse laser light has a width of 0.01 to 0.3 cm.26. A method for producing a semiconductor device, comprising:forming asemiconductor layer comprising silicon on a glass substrate; generatinga pulse laser beam in a laser oscillator; shaping said pulse laser beaminto a linear pulse laser beam having a focus line; and irradiating saidlinear pulse laser beam to said semiconductor layer while moving saidlinear pulse laser beam relatively with respect to said semiconductorlayer comprising silicon approximately perpendicularly to longitudinaldirection of said linear pulse laser beam, to crystallize saidsemiconductor layer, wherein said linear pulse laser beam has an energydistribution satisfying inequalities 0.5L1≦L2≦L1 and 0.5L1≦L3≦L1 in awidth direction thereof where L1 represents a beam width at an energydensity of 0.95, and L1+L2+L3 represents a beam width at an energydensity of 0.7 with the L2 and L3 corresponding to both side portions ofthe beam width, and a maximum energy density of said linear pulse laserbeam is assumed to be 1, and wherein said focus line approximatelycoincides with and irregular surface of said glass substrate.
 27. Themethod of claim 26 wherein said pulse laser beam has a wavelength of 248nm.
 28. The method of claim 26 further comprising crystallizing saidsemiconductor layer comprising silicon,wherein crystallinity of thecrystallized semiconductor layer is improved by said irradiating. 29.The method of claim 26 wherein said glass substrate has a temperaturefrom a temperature higher than room temperature to about 70% of a straintemperature of said glass substrate in said irradiating step.
 30. Themethod of claim 29 wherein said strain temperature is in view ofabsolute temperature.
 31. The method of claim 26 wherein said linearpulse laser beam has a width of 0.01 to 0.3 cm.
 32. The method of claim26 wherein said L1 is 0.4 mm and said L2 and said L3 are 0.25 mm.
 33. Amethod for producing a semiconductor device, comprising:forming asemiconductor layer comprising silicon on a glass substrate; generatinga pulse laser light in a laser oscillator; shaping said pulse laserlight into a laser light into a linear pulse laser light having a focusline; and irradiating said linear pulse laser light to saidsemiconductor layer while moving said linear pulse laser light relativeto said semiconductor layer comprising silicon approximatelyperpendicularly to a longitudinal direction of said linear pulse laserlight, to crystallize said semiconductor layer, and wherein said focusline approximately coincides with an irregular surface of said glasssubstrate.
 34. The method of claim 33 wherein said pulse laser light hasa wavelength of 248 nm.
 35. The method of claim 33 further comprisingthe step of crystallizing said semiconductor layer comprisingsilicon,wherein crystallinity of the crystallized semiconductor layer isimproved by said irradiating.
 36. The method of claim 33 wherein saidglass substrate has a temperature from a temperature higher than roomtemperature to about 70% of a strain temperature of said glass substratein said irradiating.
 37. The method of claim 36 wherein said straintemperature is in a view of absolute temperature.
 38. The method ofclaim 33 wherein said linear pulse laser light has a width of 0.01 to0.3 cm.
 39. A method for producing a semiconductor device,comprising:forming a semiconductor layer comprising silicon on a glasssubstrate; generating a pulse laser light in a laser oscillator; shapingsaid pulse laser light into a linear pulse laser light having a focusline; and irradiating said linear pulse laser light to saidsemiconductor layer while moving said linear pulse laser lightrelatively with respect to said semiconductor layer comprising siliconapproximately perpendicularly to a longitudinal direction of said linearpulse laser light, to crystallize said semiconductor layer, wherein saidpulse laser light has an energy variation falling within ±3% in terms of3 σ among laser pulses in said laser oscillator, and wherein said glasssubstrate has a temperature from a temperature higher than roomtemperature to about 70% of a strain temperature of said glass substratein said irradiating, and wherein said focus line approximately coincideswith an irregular surface of said glass substrate.
 40. The method ofclaim 39 wherein said pulse laser light has a wavelength of 248 nm. 41.The method of claim 39 further comprising crystallizing saidsemiconductor layer comprising silicon,wherein crystallinity of thecrystallized semiconductor layer is improved by said irradiating. 42.The method of claim 39 wherein said strain temperature is in view ofabsolute temperature.
 43. The method of claim 39 wherein said linearpulse laser light has a width of 0.01 to 0.3 cm.
 44. The method of claim20 wherein said focus line coincides with said irregular surface with arange of ±400 μm difference.
 45. The method of claim 26 wherein saidfocus line coincides with said irregular surface within a range of ±400μm difference.
 46. The method of claim 33 wherein said focus linecoincides with said irregular surface within a range of ±400 μmdifference.
 47. The method of claim 39 wherein said focus line coincideswith said irregular surface within a range of ±400 μm difference.